Are you able to openly have sex with another person you meet in the theatre or arcade?

No you can't you will get kicked out and banned

Once u go behind the curtain me and friend bought a room and we both went in there no no problem basically if you pay you stay you just have to pay for both

Then why does it say one person per room?

Private booths in the arcade are huge with plenty of room just lock the door after you fill you booth